Induction furnace



i middle portion of the winding.

' Patented Aug. 15, 1950 INDUCTION FURNACE Ludwig A. Dreyfus, Vasteras,Sweden, assignor to Allm anna

Svenska Elektriska Aktiebolaget,

Vasteras, Sweden, a Swedish corporation Application November 12, 1948,Serial No. 59,63 In Sweden November 15, 1947 v 2 Claims. 1

In many electric eddy current furnaces a substantially uniform body. forinstance a molten bath, is to be heated, and in such cases it isgenerally of minor importance if the electric induction inthe furnacedecreases towards its ends, as is usually the case. In certain cases,however, it may be of great importance that the induction be keptpractically uniform for the whole length of the furnace, especially whena common inducing winding influences a plurality of electrically more orless independent bodies which, however, are to be heated uniformly, for

instance for metallurgical reasons, or when there is only one body whichfor instance should be uniformly heat treated throughout its mass.

ing the same by 2. Fig. 1 shows inserted into the furnace a number ofbodies 3 to be heated, said bodies being only diagrammatically shown.The said bodies 3 may consist for instance of crucibles of conductingmaterial or with a conducting content, of solid bodies for instance tobe heat treated or of conducting carbonpieces for carburating atraversing gas current.

With a constant number of ampereturns per unit of length of the windingl, the flux image shown in Fig. 2 is obtained, in which the fulldrawnlines a designate magnetic lines of flux and the dotted lines D thelevel surfaces of the When using a common cylindrical winding, it isknown that the induction therein decreases to- If a uniform induction inthe wards the ends. whole operative region of the furnace is desirablewithout any particular steps being taken as regards the winding, it willtherefore be necessary to make the said operative region essentiallyshorter than the winding, so that the end portions having a reducedinduction will be in-' active. This implies, however, an imperfectutilization of the possibilities of thefurnace.

According to the present invention, a more uniform distribution of theinduction along the eddy current furnace is obtained by the provision ofan inducing winding, which as a whole is connected in parallel with acondenser for compensating the main portion of its reactive powerdemand, while certain portions of the winding, especially the ends andthe1middle winding portions can be adjusted to values other v than themean value of the winding- Thus for instance, negative reactances(capacities) may be connected in parallel to the end-portions and T apositive reactance (inductor) in parallel to'the induction which areperpendicular to the flux lines. As shown, the distance between theselevel surfaces is essentially larger towards the ends of the furnacethan in the middle height thereof, especially near the axis of thefurnace, and as the induction is proportional to the inverse value ofthe said distance, it decreases essentially towards the ends of thefurnace. In many cases it is, however, desirable that the induction ismaintained at substantially the same value at the ends of the furnace asbetween the ends, and the present invention comprises means forincreasing the number of ampereturns per unit of length of the furnacewinding towards the ends for this purpose. In the form of the inventionillustrated, condensers 4 are connected in parallel with the endportions of the furnace winding I and are preferably adjustable, while amiddle portion is connected in parallel to an inductance 5, preferablyalso adjustable. The

entire winding is as usual connected in parallel with a large condenser6 which delivers the main portion of the necessary reactive power, whilethe vactive'power is delivered by agenerator I. The result will be, thatthe number of ampereturns per unit of length of the winding, instead ofbeing normally constant as in induction furnaces, will vary according toFig; 4, in which In the accompanying drawing, Fig. I showsdiagrammatically in plan view an eddy current furnace having a pluralityof bodies to be heated and Fig. 2 shows a vertical section thereof withthe said bodies omitted but with lines of flux and level surfaces of theinduction shown in the case of the invention not being applied. Fig. 3shows I a diagram of connections for one form of the somewhat increasedand the said number of the the width of the hatched surface representsthe said number, i. e'., the number of ampereturns of the portionsparallel with condensers 'will be portion parallel to an inductance willbe somewhat reduced as compared with the portions without any parallelcurrent path. By suitable adjustment of the members 4, I, for instanceunder theinfluence of a pyrometer measuring the temperature in thediiferent portions of the furnace, the specific number of ampereturnsalong the furnace winding can be varied so that the reduction oftbemasnetic induction towards the ends of the furnace illustrated inFig. 2 is essentially compensated.

The invention may of course be combined with other means for equalizingthe induction in the furnace body, for instance with the internalinductor winding surrounded by the bodies heated by the inductiondescribed in my patent application Serial No. 59,705 of even date. Insuch case also certain portions of the internal winding, for instance inthe middle and at the ends, may be provided with parallel reactances inthe same way as the portions of thewinding I.

I claim as my invention:

1. An eddy current furnace comprising a coilshaped inductor winding, asource of high frequency current, means connecting the ends of the coilto said source, conducting means inside said winding adapted to beuniformly heated by eddy currents induced therein by said winding, avariable inductance connected in parallel with the middle portion ofsaid winding and variable capacitors connected in parallel respectivelywith each end portion of said winding beyond the points of connection ofthe inductance.

2. In a furnace as claimed in claim 1, a laminated iron shellsurrounding said winding.

LUDWIG A. DREYFUS.

4 REFERENCES crrnn The following references are of record in the tile orthis patent:

UNITED STATES PATENTS Number Name Date 1,642,198 Gerth et a1. Sept. 13,1927 1,828,291 Northrup Oct. 20, 1931 1,852,215 Northrup Apr. 5, 19321,943,802 Northrup Jan. 16, 1934 1,948,704 Fischer Feb. 27, 19341,961,621 Northrup June 5, 1934 2,452,197 Kennedy Oct. 26, 19482,490,104 Strickland, Jr. Dec. 6, 1949 FOREIGN PATENTS Number CountryDate 305,620 Great Britain Feb. 8, 1929 310,031 Great Britain Jan. 16,1930 386,387 Great Britain Jan. 19, 1933 393,958 Great Britain June 12,1933 586,822 Germany Oct. 26, 1933 764,178 France Feb. 26, 1934

